Laminates which are prepared by sandwich extrusion laminating of a base material such as various re s in film undergoing anchor coat treatment or cellophane and metallic foil or a metal-deposited resin film with a low-density polyethylene resin followed again by extrusion laminating the low-density polyethylene resin have been used as packaging films for various purposes.
About the extrusion laminating as a method of laminating which makes it possible to attain the degree to which the adhesive strength between a bonding layer and metal foil in a laminate meets practical purposes, there is a report (Convertech(8), page 36, 1991) on a method for producing laminates, in which an extrusion resin temperature of a low-density polyethylene having good workability is set, for example, at a temperature as high as 320.degree. C. or above to oxidize the resin surface within the air gap between the die and pressure roller and the bonding layer as thin as 10 to 15 .mu.m in coat thickness is allowed to adhere to metal foil's surface. However, the adhesion strength is as weak as 50 to 100 g/15 mm wide and the polyethylene elongates on unsealing of a package, being difficult to tear, because of the weak adhesion to metal. In addition, packaging of viscous, aqueous and heavy articles which requires high pressure resistance to packaging films is quite impossible.
Anchor coat treatment on metallic foil's surfaces may be considered as a means of improving the adhesion strength. However, the anchor coat treatment and extrusion laminating of a single sheet of metal foil as thin as 7 to 12 .mu.m cause tears or creases in the sheet, which makes practical work impossible. JP-A-4-368845 (The term "JP-A" as used herein means an "unexamined published Japanese patent application") reports a method of extrusion laminating of a resin containing polar groups such as an ethylene/acrylic acid copolymer resin and a method of extrusion laminating of an ethylene copolymer resin undergoing ozone treatment which is prepared by copolymerization of ethylene, an unsaturated polybasic acid and an unsaturated monomer selected from lower alkyl esters and vinyl esters of acrylic acid. However, these copolymer resins are unpreferred because of high manufacturing cost, complicated exchange of resins in an extruder and, in addition, smell of the films. When a customary CPP film or a film in which a thin film of silicon oxide is formed at least on one side of a transparent base material is sandwich laminated, the surface of the CPP film or one side of the film having a thin silicon oxide film formed undergoes anchor coat treatment prior to the laminating. In these cases, the films are unpreferred because of high manufacturing cost and smell thereof.
In these situations, a costly method using an adhesive, called "dry laminating," has been unavoidably adopted when strength is required as in packaging bags.